1. Field of the Invention
The present invention relates to an electrostatic free-fall separator for separating mixtures of substances, for example for separating mixtures of mineral raw materials, or also for the separation of plastic mixtures.
2. The Prior Art
According to the prior art, different types of free-fall separators are known, and all are operating based on the same principle. The particles to be separated are selectively oppositely electrically charged according to the triboelectric process and are permitted to free fall through a separation zone. This separation zone is defined by a pair of electrodes, and these electrodes have opposite electrical polarities brought about by applying a D.C. voltage to this pair of electrodes.
In this process, the electrodes can be designed in the form of plates, or revolving belts, or also in the form of a series of stationary or rotatably supported tubes. Due to the deflection of the particles in the electric field caused in accordance with the charge that they carry, three products are obtained. Hence, the products produced include a negatively charged material, a positively charged material, and an intermediate material. The quality of the products can be controlled by separating panels mounted at the end of the free fall drop line.
A separator operating according to the known prior art is described in Schubert, "Aufbereitung fester mineralischer Rohstoffe" (Treatment of solid raw mineral materials), Volume II, pp. 233-234, Leipzig, 1967, It is known according to DE 2,609,048, to use as electrodes revolving belts made of conductive material. A tube-type free-fall separator for separating plastic mixtures represents the state of the prior art according to DE 4,438,704. For the purpose of enhancing the purity of the separated products, provision is made according to this reference to arrange the tubes, which are known per se, in a staggered manner. Thus, these tubes are arranged with a gap in the distance relative to each other. The selectivity of the separation can be improved in this way.
In electrostatic separation, one single separator normally does not suffice for selectively separating two components to achieve a satisfactory purity. Hence, it is necessary to conduct a post separation of the products again in a two-stage or multi-stage installation. Disadvantages include having long distances of conveyance that have to be provided between the stages; the space requirement is substantial; and the investment costs rise with the increase in the number of stages. One further drawback of such multi-stage installations, which, as a rule, may comprise two and, in special cases, also three or more separators, is that horizontal conveyors are required. Thus, screw conveyors or chain conveyors are required in order to be able to convey intermediate fractions reciprocally from one separator to the other.
Two such horizontal conveyors are required in an installation with two separators. The horizontal conveyors, furthermore, substantially increase the quantity of circulating material. On the one hand, the residence time of the material in the installation increases, which may lead to increased electrical discharge of the charged particles. Electrical discharge may be caused not only by charge exchange between the electrically charged particles, but also by contact of the particles with the wall materials of the conveyor gear housing. On the other hand, the other drawback of an increased quantity of circulating material is that the achieving of the steady state equilibrium is delayed for the overall separation process.